Transcript Slide 1

Imperial College
London
3I3 Advanced Organometallics
Lectures 1 - 4
Dr. Ed Marshall, M220, RCS 1
[email protected]
Additional materials available on:
www.ch.ic.ac.uk/marshall/3I3.html
Lecture notes also available on Blackboard
3I3 Slide 1
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A question for you
What properties do you think are desirable for a catalyst?
• Cheap, robust and long-lived
• Low toxicity
• Lewis acidic metal centre – electronic unsaturation
• At least one vacant coordination site – coordinative unsaturation
• Variable oxidation states?
• Flexible metal-based frontier orbitals (energy, direction)
Large ligands (L) are often used to give coordinative (and
electronic) unsaturation. IfXL bonds to M using a flexible
L then
M M may also use a mixture of orbitals
mixture of orbitals,
to bind to a substrate.
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The next four lectures
Alkene and polyene ligands
Bonding, synthesis &
reactivity
Alkene polymerisation
Metal-carbon multiple bonds
Olefin metathesis
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Learning objectives
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By the end of lecture 4, you should be able...
1. Use simple MO theory to explain how a carbon-carbon p-cloud bonds to a metal.
2. To list methods used to synthesise metal complexes of alkenes and polyenes, and
metal-carbon multiple bonds.
3. To describe typical reactions of these complexes.
4. To appreciate how polyene ligands may respond to the electronic needs of a metal,
and how such a property is useful for catalysis.
5. To describe how cyclopentadienyl-based catalysts can be used to polymerise
alkenes.
6. To outline the most important applications of olefin metathesis.
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Assumed knowledge
Knowledge
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In order to get the most out of this course, it is worth making sure that you understand
the following concepts…
• Crystal field theory versus molecular orbital theory
• LX ligand classifications
• How to count electrons and the 18 electron rule
• Metal-alkene bonding
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3A Advanced
3I1
Advanced Organometallics
Organometallics
Section 1:
Metal-alkene complexes
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The Dewar-Chatt-Duncanson
Dewar-Chatt-Duncanson
model
Model
for metal-alkene
of Metal-Alkene
bonding
Bonding
s-component:
C-C p → empty metal orbital
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p-component:
occupied metal d → empty C-C p*
Note the similarity to CO ligands...
s-component: donation of C lone pair
p-component: backbonding into CO p*
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Best Describedversus
Metal-alkenes
as Metal-Alkenes
metallacyclopropanes
or Metallacyclopropanes?
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C-C bond distance in ethene = 1.34 Å
H atoms no
longer planar
with the C-C bond
C-C = 1.37 Å
C-C = 1.49 Å
C-C = 1.43 Å
C-C = 1.62 Å
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[Pt(C2H4)Cl3]2-
« Chem3D Embed »
versus
[Pt(C2Cl4)(PPh3)2]
« Chem3D Embed »
The impact
ConceptofOfmetal
Umpolung
coordination
- Reversal
andOf
backbonding
Polarity
on reactivity
1. Free alkenes undergo electrophilic additions,
but coordinated alkene ligands are susceptible
to nucleophilic attack
2. Backbonding reduces d+ charge and
reduces reactivity to nucleophiles
Why sp3?
d+
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No backbonding:
“metal-alkene"
sp2 carbons
With backbonding:
“metallacyclopropane"
sp3 carbons
Backbonding occurs to the p* antibonding orbital,
therefore reducing the C-C bond order
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Appendix:of
Synthesis
Synthesis
metal-alkene
& Reactivity
complexes
of Polyene LIgands
Two common methods:
1. Addition to electron poor metal centres / displacement of other L-ligands
16e-
18e-
2. Reduction of a metal complex in the presence of the neutral -ene ligand
Oxidation state: N
Oxidation state: N-2
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Synthesis of
Of metal-alkene
Metal-Alkenecomplexes:
Complexes examples
1 (a) Addition to 16 electron species:
e.g. [Ir(CO)Cl(PPh3)2] + C60
18 e-
16 e[Ir(CO)Cl(PPh3)2C60]
1 (b) Displacement of other L-type ligands:
e.g. (h5-C5H5)2Zr(PMe3)2 + C2H4
18 e-
18 e(h5-C5H5)2Zr(C2H4)(PMe3)
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Synthesis of
Of metal-alkene
Metal-Alkenecomplexes
Complexes
2. Reduction of a metal in the presence of an alkene
e.g. RhCl3 + CH3CH2OH
+ CH3CHO
Rh(III)
Rh(I)
[(nbd)Rh(m-Cl)]2
nbd = norbornadiene
e.g. (h5-C5H5)2TiCl2 + 2Na
C2H4
Ti(IV)
Ti(II)
(h5-C5H5)2Ti(C2H4)
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Reactivity of metal-alkene complexes
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Alkene ligands are often susceptible to nucleophilic attack
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Summary of section 1
1. Catalysis at a metal centre often requires a responsive metal (and therefore a
responsive ligand set)
Most useful ligands are often those that can use
different MOs to bind to a metal
2. Binding an alkene to a metal often increases its susceptibility to nucleophilic
attack
Binding any organic fragment to a metal may
activate it towards chemical modification
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